How can i add an object when the mouse is released?
here is how to detect a mouse button release and activate an actuator, using a Boolean property:

    mouse_Lclick = cont.sensors["Mouse_leftclick"] 

    if mouse_Lclick.positive :
        if not own['hold'] :
             # just pressed
             own['hold'] = True
        else :
             # button is held
    else :
        if own['hold'] :
            # button released
            own['hold'] = False
            cont.activate(actuator)  #activate the actuator